The present invention relates to an apparatus for laying underground electric cables.
In more detail, said apparatus is intended for laying electric cables in a trench, in particular when medium- or high-voltage cables and preferably high-voltage cables are concerned, of a value as high as, or greater than 150 kV, where arrangement of a mass of inert material of suitable thickness around the cable is particularly wished, which mass, in addition to performing a function of mechanical protection of the cable, also enables dissipation of the heat generated by the cable when passed through by current.
Laying of electric cables for accomplishment of medium- or high-voltage earth lines usually takes place within trenches digged in the ground.
On laying, the cable is incorporated into a mass of inert material, e.g. sand or poor concrete, usually carrying out a mechanical protection of the cable itself (see U.S. Pat. No. 4,050,261, for example, and the article by F. Donazzi, E. Occhini, A. Seppi, xe2x80x9cSoil thermal and hydrological characteristics in designing underground cablesxe2x80x9d, Proc. IEE, Vol. 126, No. 6, June ""79).
In more detail, for cable laying it is required that preliminarily a base layer of inert material should be laid down at the bottom of a trench, previously made by an excavation operation. The cable unwound from a reel is then deposited onto the base layer.
Subsequently, a covering layer of inert material is deposited so as to form, with the previously deposited base layer, a mass of inert material completely incorporating the cable. The Applicant has perceived that laying of cables carried out by manual operations, in addition to involving an important loss of time and manpower, does not offer sufficient assurances as regards achievement of the prescribed technical features in the inert material placed around the cable. The Applicant has also become aware of the fact that the thermal features of this material can vary to a great extent depending on the compacting degree given to the material during installation and that this compacting degree, if the material is laid down manually, is not very uniform and cannot be easily checked.
Document EP 0 585 188 A1 discloses an apparatus for laying underground electric cables comprising: a vehicle longitudinally movable along a trench arranged to receive at least one electric cable; a first deposition unit to form a base layer of inert material at the bottom of said trench, said first deposition unit comprising at least a first conveying duct connected at the upper part thereof with feed means for said inert material and at the lower part thereof with a downwardly-turned discharge opening; at least one guide structure, having an inlet end portion turned towards the vehicle front, to engage at least one stretch of said cable extended over the trench, and an outlet end portion disposed at a lower position and turned to the back of the first deposition unit for laying the cable on said base layer; a second deposition unit operating at the rear of the guide structure to form a covering layer of inert material upon the base layer and the cable.
In accordance with the present invention, laying of underground electric cables is made by an apparatus characterized in that the guide structure has at least one portion laterally disposed relative to said first conveying duct.
Preferably, said apparatus further comprises at least a first compacting unit operatively interposed between the first deposition unit and the outlet end portion of said guide structure for compacting the base layer deposited at the trench bottom.
In more detail, this compacting unit comprises a vibrating plate acting against the base layer by a lower surface thereof preferably having a substantially V-shaped cross-section outline. Preferably, said V-shaped outline has a rounded vertex in the form of an arc of a circle of a radius substantially corresponding to half diameter of the cable.
Advantageously, said first conveying duct and said first discharge opening have a width reduced by an amount of at least 10% relative to the width of said trench.
Preferably, the first discharge opening is delimited at the rear part thereof by an outlet edge disposed at a higher level than the lower surface of said vibrating plate. The vibrating plate, in turn, has a lead-in portion rising from said lower surface in the direction of the outlet edge of the discharge opening, and terminating at a higher level than said outlet edge.
At least one presser roller elastically acting against the cable to push it towards the base layer laid at the trench bottom may be advantageously associated with the outlet end portion of the guide structure.
The second deposition unit may advantageously comprise a second conveying duct connected at the upper part thereof with feed means for said inert material, and at the lower part thereof with a second downwardly-turned discharge opening.
Preferably, said second conveying duct and said second discharge opening have a width reduced by an amount of at least 10% relative to the trench width.
Furthermore, with the second deposition unit may be associated adjustment means operating at an outlet edge located at the rear of the second discharge opening to adjust thickness of said covering layer.
In accordance with a further aspect of the present invention, the vehicle comprises a first van and a second van removably engaged in mutual alignment and carrying the first deposition unit and second deposition unit, respectively.
Moreover, said vehicle may comprise adjustable suspension means operatively associated with respective wheels of the vehicle for modifying the height of said first and second deposition units and of said guide structure relative to the trench bottom.
Preferably, said first and second vans are mutually in engagement by at least one fluid-operated connecting actuator.